Field of the Invention
This invention relates to filter apparatus for purifying exhaust gases, which traps and collects particulates mainly composed of carbon in exhaust gases emitted from engines, such as a diesel engine and the like, and burns out said particulates.
Conventionally, the following filter apparatus has been proposed for this purpose.
The filter apparatus has a honeycomb structure having permeable partition walls, and has at least one electric heater installed on the inlet side or upstream end of said structure. The partition walls define a multiple of passages or cells along the longitudinal direction of the structure, and the ends of the passages are closed and open alternately at both ends of the structure to form inlet passages and outlet passages. The exhaust gases flow into the open inlet ends of the respective inlet passages and pass through the partition walls to flow into the outlet passages and out of the open outlet ends thereof. During such passage of the gases, carbon particulates are trapped and collected on the surface of the partition walls forming the inlet passages. Then, the trapped and collected particulates are burned by the heat of said electric heater.
In such apparatus the electric heater and the particulates trapped on the upstream portions of the partition walls are in contact with each other. Therefore, the heat of the electric heater is able to ignite the particulates. Similar apparatus is described in U.S. Pat. No. 4,276,071. This apparatus, consisting of ceramic filter materials, has a honeycomb structure. A multiple of passages is provided along the longitudinal direction of the structure, and the ends of the passages are closed and open alternately at both ends of the structure. Exhaust gas introduced into an inlet passage is filtered through small openings in the porous ceramic layer which constitutes a wall adjacent to another, i.e. an outlet, passage. After a multiple of such filtering, the exhaust gas flows out from the outlet ends of the outlet passages.
The inventors here have examined the conventional apparatus mentioned above, and noted that in the conventional apparatus it is difficult to thoroughly burn the particulates, which adher to the partition walls, by heat of the electric heater. Because, in the conventional apparatus, the electric heater is installed on the upstream end of the apparatus. On the other hand, carbon particulates in exhaust gas are mainly trapped during their passage through the partition walls which separate the multiple inlet and outlet passages. The location where the largest amount of carbon particulates is trapped is not at upstream portions of the partition walls but at place within the partition walls at a distance downstream from the upstream end of the structure. That is, only a small quantity of particulates adhere to the upstream end of the partition walls, and therefore, the amount of heat generated by the burning of these particulates is not enough to maintain a natural burning of all the particulates adhering to the entire partition walls. The heat generated in the heater is readily lost by radiation loss. Therefore, it is difficult to burn particulates adhering to the downstream portion of the partition walls. As a result, it is difficult for conventional apparatus to regenerate the downstream portion of the structure.